Tomb Raider Underworld Walkthrough and Strategy Wiki SuperGuide

Relic Locations

At the top level of the Kraken area (when coming back from thors glove
room.)

Coastal Thailand

Move the bridge after you've solved the puzzle.

Croft Manor

In a sewer near the pressure pads. You must place two weights on the
pressure pads near the exit of the sewer hall. Then the boxes will make
the gate at the exit raise but it will also raise the small sewer gates
behind you. Crawl under the sewer hole and retrieve the relic.

Southern Mexico

In Jormungandr's Pool in statue's right eye. Must have obtained Thor's
belt and returned hammer to original position.

Jan Mayen Island

Before entering the hall with swinging hammers there are two switches.
One of them leads to a Relic.

Andaman Sea

No Relic present in this level.

Arctic Sea

This is in the room before the Natla fight. You have to hit four runes
around the room to get the center waterfall to stop. Once this is done
you can swing to it.